1. Field of the Invention
The present invention relates to an adaptive antenna apparatus for use in a base station or a mobile station of, for example, a mobile communication system. In particular, the present invention relates to an adaptive antenna apparatus, which includes a plurality of pairs of antennas each having a bidirectional characteristic, and which includes a total of four or more, even number of antennas, and which takes an adaptive control on radio signals using a pair of antennas opposing to each other.
2. Description of the Related Art
FIG. 32 is a perspective view showing a configuration of a prior art adaptive antenna apparatus disclosed in the Japanese Patent Laid-Open Publication No. 10-242739. The adaptive antenna apparatus shown in FIG. 32 is constituted so that three omni-directional antennas are arranged in parallel to each other on a predetermined horizontal surface. In the adaptive antenna apparatus, the number of antennas can be reduced by setting an interval S between the adjacent omni-directional antennas to be equal to or larger than one wavelength and to be equal to or smaller than three wavelengths.
Referring to FIG. 33, two omni-directional antennas 101a and 101b are arranged in parallel to each other so as to be separated from each other by a predetermined interval S between the antennas. The omni-directional antenna 101a has a circular radiation pattern 102a, and the omni-directional antenna 101b has a circular radiation pattern 102b. It is assumed that the interval S between the antennas is half the wavelength.
FIG. 34 is a graph showing a suppressed amount of an interference wave under the adaptive control when an incident angle of a desired wave and an incident angle of an interference wave are set in the case of FIG. 33. The suppressed amount of the interference wave corresponds to an electric energy that indicates how the interference wave is suppressed under the adaptive control relative to an incidence electric energy of the interference wave under no adaptive control. It is assumed herein that the desired wave and the interference wave are equal in amplitude to each other, and that a noise having 0.001 times the amplitude of the desired wave is inputted to the adaptive antenna apparatus. In addition, the omni-directional antenna 101a is located on the Z-axis on which the X-axis intersects the Y-axis, and that the incident angle is defined as an angle from +X-axis toward +Y-axis as shown in FIG. 33. In FIG. 34, the incident angles of the desired wave and the interference waves are shown up to ±45 degrees.
As is apparent from FIG. 34, it is difficult to suppress the interference wave when a difference between the incident angle of the desired wave and that of the interference wave is equal to or smaller than five degrees. When the incident angle difference between the desired wave and the interference wave is equal to or smaller than five degrees, an effect of suppressing the interference wave is relatively small. Accordingly, this angle difference range becomes out of a control range of the adaptive antenna apparatus. However, in the other ranges, it is shown that the suppressed amount of the interference wave is relatively high, i.e., 10 dB or more.
Next, a phase shift amount of a phase shifter required for an interference wave suppression operation executed by the adaptive antenna apparatus will be examined. By way of example, the results of changing the incident angle of the interference wave when the incident angle of the desired wave is zero degree are shown in FIG. 35. A horizontal axis of FIG. 35 indicates an angle difference between the incident angle of the interference wave and that of the desired wave. A vertical axis of FIG. 35 indicates a phase difference between a phase of a weight coefficient required for the omni-directional antenna 101b and a phase of a weight coefficient required for the omni-directional antenna 101a. In this example, the angle difference between the incident angle of the interference wave and that of the desired wave up to 180 degrees is set taking into consideration the symmetry of the adaptive antenna apparatus. As can be seen from FIG. 35, the maximum phase difference in weight coefficients between the antennas 101a and 101b was 360 degrees. In other words, in order to realize this phase shift amount, it is necessary to employ a phase shifter that has a phase shift amount of 360 degrees.
Further, in the case of no interference wave, it is necessary to realize a directional characteristic having a higher gain in a desired direction. When the desired wave arrives from, for example, the +X direction, the radio signal is controlled so that the maximum radiation gain can be obtained in the +X direction. The radiation patterns as obtained at that time are shown in FIG. 36. The phase difference between the antennas 101a and 101b was 180 degrees. As is apparent from FIG. 36, a relatively large gain of 4.4 dBi can be obtained in the +X-axial direction.
By changing the interval S between the antennas from one wavelength to three wavelengths in addition to the control operation executed by the adaptive antenna apparatus, a side lobe is generated in a radiation characteristic so that one beam can be made shaper. This can realize a deep drop of the directional characteristic between the main lobe and the side lobe, and realize an excellent interference wave suppression effect by the adaptive antenna apparatus using fewer antennas than usual. As described above, it is possible to realize the adaptive antenna apparatus capable of obtaining a high gain in an arrival direction of the desired wave with no interference wave, and capable of exhibiting the interference wave suppression effect with an interference wave by a simple structure.
However, the adaptive antenna apparatus shown in FIG. 32 has the following disadvantages. As already described, when the interference wave is present, it is necessary that the phase difference between the weight coefficients of signals inputted to the respective antennas is as large as 360 degrees so as to cover all directions on the horizontal surface. An ordinary phase shifter including a 90-degree hybrid circuit and a variable amplifier can in principle obtain a phase shift of only up to 90 degrees. Further, it is noted that a commercially available phase shifter using a diode has a phase shift amount of about 100 degrees. Accordingly, in order to realize the phase shift amount of 360 degrees, it is necessary to connect these phase shifters at multiple stages. This makes the circuit scale of the adaptive antenna apparatus larger, and this leads to making it difficult to reduce the size of the adaptive antenna apparatus. Moreover, as shown in FIG. 32, it is necessary to provide amplitude adjusters and phase shifters for adjusting amplitudes and phases as many as the antennas. Since the phase difference and the amplitude difference between the antennas are necessary for the control by the adaptive antenna apparatus, the number of amplitude adjusters and phase shifters can be reduced by one from the number of antennas. However, with the configuration of the prior art adaptive antenna apparatus, the number of shifters that can be reduced is limited up to one. In addition, as shown in FIG. 36, in the case of no interference wave, a radio wave is radiated similarly in a −X-axial direction opposite to the X-axial direction that is the direction of the desired wave. Further, since the interval S between the antennas is changed from one wavelength to three wavelengths, the size of the adaptive antenna apparatus cannot be reduced. Accordingly, it should be said that the structure of the prior art adaptive antenna apparatus which has a limit to a gain improvement due to such a wasteful use of radiation power is inappropriate.